In normal use, viruses, Trojans and you cannot infect or damage your Ubuntu from within a Windows VM.
NEVER SAY NEVER
There are exceptions. The biggest risk is in sharing a partition with critical system files, or other important files between the VM and real machine. If you must share a partition, create one for that purpose and back the files up or move them out as quickly as you use them.
I was never successful creating and sharing a partition for use between XP and Ubuntu, so I set up and use DropBox for my file sharing and make daily backups of my DropBox folder from Ubuntu each night.
I set up and used XP VMs in DropBox for over 5 years. I only installed XP SP3 and NO UPDATES! I never used anti-virus software except Windows Defender and an on-line scanner that I occasionally use just to double check. I never had a virus and don't expect I ever will.
Here is how I protect myself:
I created one VM and installed XP (I now use Windows 7) and installed a few programs I would use on any Windows machine such as LibreOffice, DropBox and Skype. Then, instead of cloning the drive, I create a new VM "linked" to the original drive. This creates separate paths, or instances, of the drive that are unique. Each machine has a specific purpose. Most are used for only one piece of software such as my Bible Software. Others have multiple programs such as the one I use for video editing. I have three pieces of software I use to edit videos.
I NEVER browse the net or do downloads from Windows. I do that from Linux. On rare occasions I find a need to visit a specific website, but then I have reason to trust the site.
I always close the VM when I am not actively using it. When I do share files, I open the files in DropBox and save them there. At night, Ubuntu backs up all the files in DropBox to a dated folder in my backup drive.
If I ever find an infection or have a problem, disaster, etc. in Windows, I just roll back to the previous image of the machine and keep moving. All my important information is saved in DropBox or it's backup.